Crystal structure of the high-pressure phase Mg4(MgAI)AI4[Si6O21/(OH)7]: an analogue of sursassite
Gottschalk, Matthias; Fockenberg, Thomas; Grevel, Klaus-Dieter; Under, Bernd W.; Wirth, Richard; Schreyer, Werner; Maresch, Walter V.
European Journal of Mineralogy Volume 12 Number 5 (2000), p. 935 - 945
published: Oct 4, 2000
manuscript accepted: Apr 3, 2000
manuscript received: Jun 17, 1999
ArtNo. ESP147051205002, Price: 29.00 €
Abstract The crystal structure of the synthetic high-pressure phase Mg4(MgAl)A4[Si6O21/(OH)7], presently referred to as MgMgAI-pumpellyite in the literature, has been determined by the Rietveld method, and found to be isostructural with sursassite rather than pumpellyite. This phase is therefore a slightly modified Mg-analogue of sursassite Mn42+Al2AI4[Si6o22/(OH)6], with Mg for Mn and one Al replaced by Mg+H. For material synthesized at 5 GPa and 600°C, the following monoclinic cell-dimensions (Z = 1, space group = n/m) were derived: a = 8.5424(8) Å, b = 5.7117(3) Å,c = 9.6484(6) Å,ß = 108.298(4)°, V = 447.0(1) Å3. Besides domain boundaries the material was well crystallized and HRTEM investigations did not suggest any stacking disorder along  or sursassite-pumpellyite intergrowth.